This invention relates to a method and apparatus of NMR imaging, namely a method of tomography utilizing the NMR phenomenon, and particularly to a method and apparatus for imaging with the intention of eliminating the influence of respiratory motion.
An NMR imaging apparatus takes 2-20 minutes for taking an image, and imaging of thoraxic and abdominal parts is influenced by breathing, resulting in blurring and artifact of image. For overcoming this problem, there are two (or in more fine classification three) approaches. One is to reduce the repetition time of imaging so that imaging completes within a time length in which the patient can hold breathing. However, this method has a problem of different quality of image from the case of usual imaging with a longer repetition time, due to the influence of relaxation time of spin. The second approach, which eliminates the influence of breathing while expending an equal repetition time employs a means of synchronization or modification of measured data through the detection of respiratory motion. The method of synchronization through the detection of respiratory motion unfavorably involves complicated control and extended measuring time, whereas the method of modification of measured data can readily be carried out on a software basis without any prolongment of measuring time. The method of measured data modification by detecting the respiratory motion is discussed in the proceeding of SMRM (1985), pp. 962-963.
In measuring the NMR signal, an image measurement signal is produced from the first echo signal, and a projection data measurement signal is produced from the second echo signal. The Fourier transformation for the second echo signal yields projection data 400 shown in FIG. 4. By detecting the edge portion (b) 402 or (c) 403 of the projection data, the state of breathing is known. The respiratory motion is defined between the edge portion and the highest point a among quiescent points whose positions do not move conceivably. It is conceived that the respiratory motion is a linear expansion and contraction, and the image measurement signal is subjected to such a linear transformation.
Although the foregoing prior art assumes that the motion in the body caused by breathing is a linear expansion and contraction between the quiescent point and edge portion, the actual respiratory motion in the abdominal part is not necessarily a linear expansion and contraction, and this method for motion modification can do little for eliminating the degradation of image quality attributable to breathing.